1. Field of the Invention
The present invention relates to a head for checking linear dimensions of parts in machine tools or measuring machines, with a support structure, a movable arm-set including an. arm carrying a feeler for contacting the part to be checked, a bias device arranged between the support structure and the movable arm-set, a first constraining system having a structure with rotational symmetry about a first geometrical axis and a second constraining system of a type different from that of the first constraining system, the first and the second constraining system being arranged between the support structure and the movable arm-set, for eliminating, under the action of the bias device, the six degrees of freedom of the movable arm-set, the degrees of freedom eliminated by the first constraining system including the translation of the movable arm-set along said first geometrical axis, and a detecting device for providing a signal depending on the position of the movable arm-set.
2. Description of the Related Art
Contact detecting heads, or touch trigger probes, and measuring heads are used in co-ordinate measuring machines and machine tools, particularly machining centres and turning machines, for checking machined or being machined parts, tools, machine tables, etc. These heads generally include a movable arm-set with an arm carrying one or more feelers, a biasing device for urging the arm-set against a support structure, and a detecting device with one or more switches, or other devices for providing a triggering signal, or with position transducers.
In connection with contact detecting heads, the displacement of the feeler due to the contact with the part causes triggering of the detecting device, that in turn controls the reading of transducers associated with the machine slides, that provide measurement values with respect to a reference position or origin.
Basic requirements for these heads are the repeatability i.e., the correspondence among determined positions of the feeler and the triggering of the detecting device or, for the measuring heads, the values of the signals of the transducers of the head, reliability, sturdiness, small overall dimensions and a limited cost.
An indicative value of the acceptable repeatability error for these heads is of 1 xcexcm, or less.
Some of the heads are anisotropic with respect to triggering of the detecting device as a consequence of transversal displacements of the feeler. On this regard, it is pointed out that normally the transversal displacements are not purely translational, but correspond, for example, to rotational displacements of the movable arm-set. However, in view of the small entity of the feeler displacements, it is customary to refer to feeler displacements along the directions of the transversal axes.
Substantially, the concept of anisotropy consists in that, as the direction of the transversal displacement changes, the triggering of the detecting device takes place in correspondence with different eccentricity values of the feeler with respect to the longitudinal geometric axis of the head.
An example of a head strongly anisotropic is illustrated in FIGS. 1 to 3 of U.S. Pat. No. 4,153,998.
Another anisotropic head is described in the Japanese patent application laid-open with number 63-263406.
Other known heads, as those described in U.S. Pat. No. 5,299,360, GB-A-2,205,650 and inventor""s certificate SU-A-1516786, and some of those described in U.S. Pat. No. 5,146,691, in particular the head shown in FIGS. 1 to 3 of the latter patent, are, at least conceptually, of isotropic type.
Further differences among the conventional heads relate to the systems for constraining the movable arm-set to the support structure.
As well known, a rigid body free in space has six degrees of freedom that, with reference to a Cartesian co-ordinate system, consist in the possibility of performing translations along the axes X, Y and Z and rotations about the same axes.
As an example, the movable arm-set of the head described in U.S. Pat. No. 5,299,360 has, in the absence of forces acting on the feeler, two degrees of freedom (possibility of rotating about the X and Y axes). The movable arm-set of the head shown in FIGS. 1 to 3 of U.S. Pat. No. 4,153,998, that is coupled to the support structure through a constraining system constituted by the so-called Boys"" joint (three cylindrical elements fixed to the arm-set and three pairs of balls fixed to the support structure), does not feature, still in absence of forces acting on the feeler, any degree of freedom.
The systems constraining the movable arm-set with respect to the support structure can feature force closure and/or deformations.
In the kinematic constraint systems (shape constraints with force closure), the degrees of freedom (one or more) are eliminated under the action of forces, for example elastic forces, that xe2x80x9cclosexe2x80x9d conceptually rigid elements of the system, maintaining them into contact. When the feeler is biased by forces having values higher than the above mentioned elastic forces, it displaces and eliminates the concerned constraint, without causing (conceptually, namely with reference to ideal rigid bodies) deformations of the constraint system. An example of a head with constraint system with full force closure is the already mentioned head shown in FIGS. 1 to 3 of U.S. Pat. No. 4,153,998, that features a Boys"" joint, i.e. a system typically featuring an anisotropic structure.
On the contrary, in connection with constraints featuring deformations, like the leaf spring of the head described in GB-A-2,205,650, the displacement of the feeler occurs due to the elastic deformation of one or more elements of the constraint system.
The constraint systems featuring deformations involve some problems, for example because the deformations must be limited, in order to avoid that they become permanent, and because these systems normally feature poor ruggedness. In particular, drawbacks of this type arise when the feeler must have the possibility of performing large movements, for example in contact detecting heads used in machine tools requiring very fast measurement cycles. In similar situations, the head must allow large values of the xe2x80x9cextra-travelxe2x80x9d of the feeler after the generation of the triggering signal.
Further drawbacks occurring in some conventional heads derive from the fact that the constraint systems used in these heads would give rise to situations of over-constraint, because the constraints would be in excess with respect to those needed to eliminate one or more degrees of freedom of the movable arm-set. For example, if a head features two independent constraints with respect to the axial translation of the movable arm-set, it is necessary to add a de-coupling device for eliminating the effect of one of the constraints on the feeler, in order to avoid instability problems and thus poor repeatability of the head. The elimination of the superabundant constraints involves complications in the head structure and other drawbacks (use of de-coupling elements based on deformations, arm-sets consisting of a plurality of mutually movable elements, need of a plurality of biasing devices). Similar situations occur in the heads described in the already mentioned patent U.S. Pat. No. 5,146,691.
An object of the present invention is to provide a contact detecting head, or a measuring head, permitting advantageous compromises, with respect to the prior art, in connection with the characteristics of repeatability, sturdiness, isotropy, reliability, simplicity of structure, possibility of use both in metrology department and in workshop environment, or some of these characteristics.
A favourable compromise of relatively general character is obtained by a head of the type initially mentioned, that corresponds to that described in the patent application GB-A2,205,650, in which the first constraining system and the second constraining system are totally with force closure.
More generally, the invention relates to characteristics regarding the structure and the functions of the constraining systems, the bias device and the detecting device, considered singly or in combination with other characteristics.